Complement activation is a potent mediator and diagnostic indicator of inflammation and rejection in solid organ transplants. Mechanistically, a series of effector molecules in the complement cascade mediate pro-inflammatory functions that can account for chemotaxis and activation of cells of the innate immune system, such as granulocytes and monocytes. Simultaneously, many of these same complement mediators activate and disrupt the endothelial cell interface between the recipient and the transplant, in addition, complement can stimulate B and T lymphocytes of the adaptive immune system. Complement also participates in the non-inflammatory clearance of apoptotic cells. Therefore, the complement cascade can be activated by multiple mechanisms and various components of complement can modulate the response to transplants in different directions.
To date no xenotransplantation trials have been entirely successful due to the many obstacles arising from the response of the recipient's immune system. This response, which is generally more extreme than in allotransplantations, ultimately results in rejection of the xenograft. There are several types of xenograft rejection: hyperacute rejection and acute vascular rejection which are due to the response of the humoral immune system and cellular rejection and chronic rejection which is based on cellular immunity.
Hyperacute rejection is mediated by the binding of xenoreactive natural antibodies (XNAs) to the donor endothelium causing activation of the human complement system, the major epitope XNAs target is the α-gal epitope (galα1-3galβ1-(3)4glcnac-r) which is abundantly present on glycolipids and glycoproteins of non-primate mammals and new world monkeys due to glycosylation by the enzyme α1,3galactosyltransferase (α1,3gt). In humans, apes and old world monkeys, this epitope is absent because the α1,3gt gene was inactivated in ancestral old world primates. Instead, humans, apes and old world monkeys produce the anti-gal antibody, which specifically interacts with α-gal epitopes and which constitutes ˜1% of circulating immunoglobulins. The immune response due to α-gal epitopes is an important factor in xenogenic organ/tissue transplant failure. The elimination of the interaction between the natural anti-gal antibodies and α-gal epitopes on the xenografts is a prerequisite to the success of xenografts in humans. Anti-gal has functioned as an immunological barrier, preventing the transplantation of pig organs into humans, because anti-gal binds to the α-gal epitopes expressed on pig cells. It is known from the prior art to generate α1,3gt knockout pigs that lack α-gal epitopes which has resulted in a partial elimination of this immunological barrier and hence partially overcome hyperacute rejection, however transgenic production of pigs is both expensive and time consuming. Moreover, a low but potentially significant levels of galα(1,3)gal are still expressed on the tissues of α1,3gt knockout animals, which indicates that another glycosyltransferase is involved in the synthesis of this epitope (Milland et al Immunol. Cell Biol 2005, 83, 687-693). It is also known from the prior art that green coffee bean α-galactosidase and recombinant human α-galactosidase can remove α-gal epitopes from the cell surface of tissues such as the porcine aortic valve and pericardial tissue, but enzymatic treatments have limitations not only on cost effectiveness but can also affect the histoarchitecture of the tissue and leave undesirable enzymatic residues. In order to overcome hyperacute rejection it is also known to inhibit the recipient's complement cascade through the use of cobra venom factor (which depletes C3), soluble complement receptor type 1, anti-C5 antibodies, or C1 inhibitor (C1-INH). Disadvantages of this approach include the toxicity of cobra venom factor, and most importantly these treatments would deprive the individual of a functional complement system. As regards acute vascular rejection, this type of rejection occurs in discordant xenografts within 2 to 3 days, if hyperacute rejection is prevented. The process is much more complex than hyperacute rejection and is currently not completely understood however, if hyperacute and acute vascular rejection are avoided accommodation is possible, which is the survival of the xenograft despite the presence of circulating XNAs. The graft is given a break from humoral rejection when the complement cascade is interrupted, circulating antibodies are removed, or their function is changed, or there is a change in the expression of surface antigens on the graft. This allows the xenograft to up-regulate and eventually express protective genes.
It is therefore desirable that acellular xenogeneic vascular matrices do not activate complement. It is also desirable that acellular xenogeneic vascular matrices are devoid of antigenic components and especially α-gal epitopes in order to mitigate antibody-mediated inflammatory reactions. It is also desirable to provide an alternative and more cost effective method of preparing small and medium diameter vascular products for bypass surgery. It is also desirable to provide acellular xenogeneic vascular matrices for transplantation that are devoid of residual α-galactosidase enzymatic residues.